Pacemakers for generating artificial stimulating pulses for the heart, and which may be implanted in the body, are well known. Originally the electrical circuitry for such pacemakers was of analog design, but in recent years digital circuitry has also been employed. A digital approach to pacemakers has led to the evolution of programmable pacemakers--pacemakers having paramaters such as pulse rates which are adjustable (programmable) once the pacemaker has been implanted. Programmable pacemakers are described in, for instance, British Specification Nos. 1,385,954 and 1,398,875. Such pacemakers have circuitry to detect and decode signals transmitted outside the body and alter the program accordingly. In British Specification No. 1,385,954 (claiming priority based on U.S. Ser. No. 141,694, in turn a parent of U.S. Pat. No. 3,805,796 to Tenz) the programming is accomplished by means of a magnetic field which is sensed by a magnetic reed switch; the opening and closing of the switch provides programming pulses to a program store. In British Specification No. 1,398,875 (based on U.S. Pat. No. 3,833,005 to Wingrove) the programming is by means of radio freuency transmission and reception.
It is clearly of paramount importance that the stored program in such equipment is only altered when desired and not, for example, in response to electrical noise generated in proximity to the pacemaker. If the pacemaker is arranged to receive and decode radio frequency signals for changing the stored program then clearly precautions must be taken to avoid any undesired radio frequency transmissions from altering the program.